New working conditions prompted strikes by postal staff last year
Royal Mail managers have voted to reject the company's proposed pension changes, increasing the threat of strike action.
Unite advised staff to reject the plans and 85% of those who voted in a ballot agreed with the union.
Royal Mail plans to cut the value of its pension scheme for new and current members and raise the retirement age.
Unite said it would now consider a ballot for industrial action if the issue was not dealt with by Royal Mail.
Unite national secretary, Paul Reuter, said: "This is a massive rejection of Royal Mail's decision to reduce the future pensions of thousands of working people.
"It would be absolute folly if the business were to ignore their loyal staff."
But he stressed that the union did want to rush into a strike.
The main trade union at the Royal Mail, the Communication Workers Union (CWU), is holding a workplace ballot of its own members to judge their opinion.
This ends on 25 March.
But a Royal Mail spokesman said the proposed changes were essential to deal with a deficit of £3.4bn in the pension scheme.
"After discussions on pension change which began nearly a year ago, the unions, including Unite, signed a deal with Royal Mail fully agreeing the essential changes to the pension plan which are taking effect on 1 April.
"Royal Mail is determined to ensure that everyone in the company has the best possible pension that the company can afford but we have no alternative but to reform the pension plan," he added.